1. Field of the Invention
The present invention relates to an apparatus and a method of washing objects such as semiconductor wafers.
2. Description of the Related Art
In the manufacture of semiconductor devices, semiconductor wafers are washed by washing machines, whereby process solutions, impurities, and the like are removed from the wafers. A washing apparatus of this type washes wafers with ammonia water, with water, and with fluoric acid, thereby cleaning the surfaces of the wafers.
Some problems are inherent in the washing apparatus. First, the liquid in which a wafer has been washed with remains on the surface of the wafer when the wafer is washed with another solution, making it difficult to wash the wafer thoroughly. Second, the washing liquids, for example, ammonium and fluoric acid, react with each other, forming salt which eventually remain in the form of particles on the surface of the wafer and inevitably reduces the yield of semiconductor devices. Third, when the wafer comes out of contact with any washing liquid and is exposed to air, it is dried and may have a stain on its surface, which will impair the outer appearance of the wafer and degrade the electric characteristic thereof.
Generally it is desirable that each washing liquid be maintained pure and clean in order to increase the washing efficiency. To this end, the washing liquid discharged from the washing tank is passed through a filter, thereby removing impurities or other liquid as much as possible, and the washing liquid thus cleaned is supplied back into the tank and used again, as is disclosed in Published Unexamined Japanese Patent Application No. 62-94939. The filtration, however, cannot clean the washing liquid completely.